Phonon emission and absorption in the fractional quantum Hall effect

We investigate the time dependent thermal relaxation of a two-dimensional electron system in the fractional quantum Hall regime where ballistic phonons are used to heat up the system to a non-equilibrium temperature. The thermal relaxation of a 2DES at $\nu=1/2$ can be described in terms of a broad band emission of phonons, with a temperature dependence proportional to $T^4$. In contrast, the relaxation at fractional filling $\nu=2/3$ is characterized by phonon emission around a single energy, the magneto-roton gap. This leads to a strongly reduced energy relaxation rate compared to $\nu=1/2$ with only a weak temperature dependence for temperatures 150 mK $< T <$ 400 mK.Comment: 4 pages, 3 figures; 14th International Conference on High Magnetic Fields in Semiconductor Physics, September 24-29, 2000, Matsue, Japa

Lifting of the Landau level degeneracy in graphene devices in a tilted magnetic field

We report on transport and capacitance measurements of graphene devices in magnetic fields up to 30 T. In both techniques, we observe the full splitting of Landau levels and we employ tilted field experiments to address the origin of the observed broken symmetry states. In the lowest energy level, the spin degeneracy is removed at filling factors $\nu=\pm1$ and we observe an enhanced energy gap. In the higher levels, the valley degeneracy is removed at odd filling factors while spin polarized states are formed at even $\nu$. Although the observation of odd filling factors in the higher levels points towards the spontaneous origin of the splitting, we find that the main contribution to the gap at $\nu= -4,-8$, and $-12$ is due to the Zeeman energy.Comment: 5 pages, 4 figure

Uplift of Central Mongolia Recorded in Vesicular Basalts

Epeirogenic histories of highland areas have confounded earth scientists for decades, as there are few sedimentary records of paleoelevation in eroding highlands. For example, mechanisms that have led to the high elevations of the Hangay Mountains in central Mongolia are not clear, nor is it well understood how the epeirogenic history of central Mongolia is connected to that of a broader region of high elevation that extends hundreds of kilometers to the north, east, and west. However, preserved basaltic lava flows record paleoelevation in the size distributions of vesicles at the tops and bottoms of flow units. As an initial step toward better understanding the tectonics of this part of Asia, we collected and analyzed samples from several basaltic lava flows from throughout the Hangay Mountains to use as a paleoaltimeter on the basis of lava flow vesicularity. Samples were dated and scanned with x-ray tomography to provide quantitative information regarding their internal vesicle size distributions. This yielded the pressure difference between the top and bottom of each flow for paleoelevation calculation. Results suggest that the Hangay Mountains experienced uplift of more than 1 km sometime during the past 9 m.yr. The magnitude of uplift of the Hangay, in addition to the composition of its lavas, the geomorphology of the region, its drainage pattern history, and other proxies, bears on possible mechanisms for uplift of this part of central Asia

Long-Term Efficacy and Safety of Chronic Globus Pallidus Internus Stimulation in Different Types of Primary Dystonia

Background: Deep brain stimulation (DBS) of the globus pallidus internus (GPi) offers a very promising therapy for medically intractable dystonia. However, little is known about the long-term benefit and safety of this procedure. We therefore performed a retrospective long-term analysis of 18 patients (age 12-78 years) suffering from primary generalized (9), segmental (6) or focal (3) dystonia (minimum follow-up: 36 months). Methods: Outcome was assessed using the Burke-Fahn-Marsden (BFM) scores (generalized dystonia) and the Tsui score (focal/segmental dystonia). Follow-up ranged between 37 and 90 months (mean 60 months). Results: Patients with generalized dystonia showed a mean improvement in the BFM movement score of 39.4% (range 0 68.8%), 42.5% (range -16.0 to 81.3%) and 46.8% (range-2.7 to 83.1%) at the 3- and 12-month, and long-term follow-up, respectively. In focal/ segmental dystonia, the mean reduction in the Tsui score was 36.8% (range 0-100%), 65.1% (range 16.7-100%) and 59.8% (range 16.7-100%) at the 3- and 12-month, and long-term follow-up, respectively. Local infections were noted in 2 patients and hardware problems (electrode dislocation and breakage of the extension cable) in 1 patient. Conclusion: Our data showed Gpi-DBS to offer a very effective and safe therapy for different kinds of primary dystonia, with a significant long-term benefit in the majority of cases. Copyright (c) 2008 S. Karger AG, Base

Tuning the onset voltage of resonant tunneling through InAs quantum dots by growth parameters

We investigated the size dependence of the ground state energy in self-assembled InAs quantum dots embedded in resonant tunneling diodes. Individual current steps observed in the current-voltage characteristics are attributed to resonant single-electron tunneling via the ground state of individual InAs quantum dots. The onset voltage of the first step observed is shown to decrease systematically from 200 mV to 0 with increasing InAs coverage. We relate this to a coverage-dependent size of InAs dots grown on AlAs. The results are confirmed by atomic force micrographs and photoluminescence experiments on reference samples.Comment: 3 pages, 3 figure

High frequency conductivity in the quantum Hall effect

We present high frequency measurements of the diagonal conductivity sigma_xx of a two dimensional electron system in the integer quantum Hall regime. The width of the sigma_xx peaks between QHE minima is analyzed within the framework of scaling theory using both temperature T=100-700 mK and frequency f <= 6 GHz in a two parameter scaling ansatz. For the plateau transition width we find scaling behaviour for both its temperature dependence as well as its frequency dependence. However, the corresponding scaling exponent for temperature kappa=0.42 significantly differs from the one deduced for frequency scaling (c=0.6). Additionally we use the high frequency experiments to suppress the contact resistances that strongly influences DC measurements. We find an intrinsic critical conductivity sigma_c=0.17e^2/h, virtually independent of temperature and filling factor, and deviating significantly from the proposed universal value 0.5e^2/h.Comment: Proceedings of the '14th international conference on high magnetic fields in semiconductor physics' (Semimag-2000) in Matsue, Japa

Scaling of the quantum-Hall plateau-plateau transition in graphene

The temperature dependence of the magneto-conductivity in graphene shows that the widths of the longitudinal conductivity peaks, for the N=1 Landau level of electrons and holes, display a power-law behavior following $\Delta \nu \propto T^{\kappa}$ with a scaling exponent $\kappa = 0.37\pm0.05$. Similarly the maximum derivative of the quantum Hall plateau transitions $(d\sigma_{xy}/d\nu)^{max}$ scales as $T^{-\kappa}$ with a scaling exponent $\kappa = 0.41\pm0.04$ for both the first and second electron and hole Landau level. These results confirm the universality of a critical scaling exponent. In the zeroth Landau level, however, the width and derivative are essentially temperature independent, which we explain by a temperature independent intrinsic length that obscures the expected universal scaling behavior of the zeroth Landau level